Websocket error when trying to use Caddy with Wekan

I’m getting this websocket error when trying to run Caddy version 0.9 with Wekan:

8e71878….js?meteor_js_resource=true:77 WebSocket connection to 'wss://sub.domain.org/sockjs/263/f5m9n8ej/websocket' failed: Error during WebSocket handshake: Unexpected response code: 400

With this Caddy config:

sub.domain.org {
        proxy / localhost:8080 {

How to fix?

Wekan is running in Docker container on server.

It seems that this error is because Websocket is not currently enabled in Google Cloud.

Does somebody know how to enable Websocket in Google Cloud?